1. Field of the Invention
The present invention relates in a general way to an optical system for observing an object to be inspected with the naked eye or through a videomonitor and/or photoelectrically detecting an image of the object or a series of light intensity information representing the object. More particularly, the present invention relates to an optical system usable with an apparatus for manually or automatically aligning a wafer with a photomask having a semiconductor integrated circuit pattern, an apparatus for detecting a mark on a photomask and a mark on an exposure device to place the photomask at a correct position in a photomask stage, or an apparatus for inspecting defects on a surface of a semiconductor wafer.
2. Description of the Prior Art
Recently, a semiconductor integrated circuit has been miniaturized, and the tendency of the miniaturization still continues. To meet the tendency, optical projection systems have been proposed as one of the means usable with a semiconductor aligner. The optical projection systems are such that an electrostatic circuit formed in a photomask or on a reticle is transferred onto a wafer through a lens system and/or reflector system with high resolution power. Also, other techniques, such as contact exposure and proximity exposure, are utilized.
Those systems require that the alignment work is made automatic, since the photomask and the wafer have to be aligned before the light application, and also since a great number of wafers have to be processed at a high speeds. Many proposals have been made to achieve this, for example, U.S. Pat. No. 4,251,129 assigned to the Assignee of the present application.
To achieve this automation, the above mentioned U.S. patent proposes that spatial frequency filtering is effected on the pupil plane, or a plane conjugate with the same, of the alignment optical system. In those types of systems, it is also important that the performance of the imaging of the pupil is ensured, in addition to the imaging of the object (alignment mark). However, the optical system which has to satisfy those dual requirements, would have difficulty in changing the length of the optical path of the alignment optical system. In other words, when the length of the optical path is to be changed in use, it would be difficult to satisfy both of the above described two requirements.
In an aligning apparatus, such a change in the optical path length is required. This will be explained in detail in conjunction with FIG. 1, wherein a photomask MA is conjugate with a wafer WA with respect to a projection lens PL. The optical system further includes objective lenses L1 and L2, beam splitters M1 and M2 having a half-mirror and relay lenses L3 and L4, which each have a function of forming an image with the rays from each of the objective lenses L1 and L2. Although each of the lenses is shown as a single lens, it may have plural lens elements. The other optical elements are not shown in the Figure for the sake of simplicity. The optical system further includes illumination light sources LS1 and LS2, and condensing lens L5 and L6 which form images of the light sources LS1 and LS2 on the pupil plane of the objective lenses L1 and L2, respectively, to illuminate the object by Kohler illumination.
The objective lens unit comprising the objective lens L1, the beam splitter M1, condensing lens L5 and the illumination light source LS1, and the other objective lens unit comprising the objective lens L2, the beam splitter M2, condensing lens L6 and the illumination light source LS2, are each movable as a unit toward and away from each of the imaging lenses L3 and L4, thus allowing the objective lenses L1 and L2 to move to such positions as to be expected to face the respective alignment marks on the mask, so that the alignment marks are observed while the objective lenses are placed in the positions shown by broken lines L1' and L2', for example.
When a wafer chip of a different size is loaded, the position of the alignment marks also changes necessarily, so that the objective lenses have to be displaced. The change in the position of the objective lens, L1 or L2, leads to the change in the distance therefrom to the imaging lens L3 or L4, resulting in the optical length change.
In another part of the manufacturing process which has been substantially automated, the transportation and loading of photomasks are automated, and the masks have to be loaded with extreme positional accuracy, so that another alignment operation is needed. It is possible to utilize the mask-wafer alignment optical system for such other alignment. In this case, the objective lens is moved to face the setting mark with the result of a change in the optical path length.
FIG. 2 shows the optical system diagrammatically, wherein the optical system faces a surface O of an object, e.g., a mask. The optical system includes an objective lens L1 and imaging lens L3. The image of the object is formed on the imaging surface I. Depicted by reference character P is a pupil. The rays from a point on the object surface O are made parallel by the objective lens L1 and imaged on the imaging surface I by the imaging lens L3. The broken lines show the imaging of the pupil. The illumination optical system has been omitted for the sake of simplicity of explanation.
The displacement or the translational movement of the objective lens, explained with FIG. 1, corresponds to the change, in FIG. 2, of the distance l between the objective lens L1 and the imaging lens L3 while maintaining the distance l.sub.0 between the objective lens L1 and the object O constant, for example, by moving the imaging lens L3 toward the objective lens L1 to reduce the distance l. On this occasion of the change in the distance l, the imaging of the pupil shown by the broken lines is not maintained, even if the imaging of the object can be easily maintained. This results in the degraded performance of the spatial frequency filtering.